lol yah , its a tropical mom and pop store i was looking at last.... and they have to mark up to stay alive in the market, only thing they got going for them is theyre primarily a salt-water fish store...

I have three snails, and I find them so funny. They do have a large bioload though. From what I understand, ghost shrimps have little bioload.
I've never had shrimp though, so I don't know much about them.
If you did get a large population of shrimp, I think your betta would eat some of them...

You should be able to prevent population growth of both the snails and the ghost shrimp. The snails lay large and vary noticeable egg sacks at the surface of the water that can easily be removed. Brine shrimp larva require brackish water to survive and grow. So you don't need to worry about that.

2.99 for a ghost shrimp is really expensive, they are only 35 cents at my LFS. They don't really live long (1 year usually) and have a high mortality rate so for 2.99 a piece I'd go for the snails.